In many wireless communications systems, the power amplifiers (PA) in the transmitter are required to be very linear, in addition to being able to simultaneously amplify many radio channels (frequencies) spread across a wide bandwidth. They also have to do this efficiently, in order to reduce power consumption, need for cooling and to increase the lifetime of the amplifiers. The linearity is required to be good since non-linear amplifiers would cause leakage of interfering signal energy between the channels.
The amplitude probability density of a mix of sufficiently many independent radio frequency (RF) channels, or of a multi-user CDMA (Code Division Multiple Access) signal, tends to be close to a Rayleigh distribution having a large peak-to-average power ratio. In order not to compress the waveform the amplifier must be operated with the average power backed off from the peak power of the amplifier. Since a conventional RF PA (Power Amplifier), especially class B, generally has an efficiency proportional to its output amplitude, its average efficiency is very low for such signals.
Several methods to increase the efficiency of the RF power amplifiers when transmitting signals with large amplitude variations have been proposed. Composite power amplifiers are common solutions. Two widely used methods are Chireix outphasing and the Doherty technique. Both of these well-known methods are emanating from the 1930's and are briefly presented in basic power amplifier literature, e.g. [1]. In more recent years, Meinzer has proposed another solution sometimes referred to as PAMELA, see e.g. [2].
In Chireix outphasing, amplitude modulation is obtained by combining two phase-modulated constant-amplitude signals through a combiner network (c.f. e.g. FIG. 1). The efficiency of a Chireix amplifier can have two peaks at different output voltages, compared with one for a conventional class B amplifier. The location of the extra peak is selected by the design of the combining network.
The Doherty amplifier (c.f. e.g. FIG. 2) basically consists of two amplifying elements, where the first one operates linearly through the entire range, while the second only operates during signal peaks. The efficiency curve exhibits an extra peak as compared with a conventional class B amplifier. The voltage of this extra peak is determined by the relative sizes of the amplifiers together with the output network.
The PAMELA technique (c.f. e.g. FIG. 3) uses an uneven number of amplifiers connected to a load via impedance conversion circuits. The sum of the currents will generate a voltage over the load. The voltage on each amplifier is kept constant and as high as possible, except possibly at very low output voltages. By selecting compensation reactances of pairs of amplifiers, an extra efficiency peak (as compared with class B amplifiers) can be achieved at a chosen output voltage.